1. Field of the Invention
The present invention relates to a semiconductor device such as a freewheeling diode or converting diode, mounted together with a switching device in a power conversion device, and used for reflux when the switching device is inhibited.
2. Description of the Background Art
A diode is such that, even when the applied voltage polarity is changed from forward to reverse, reverse current flows for a while rather than the current being immediately cut off. This is because a current generated by an excess of electron-hole carriers including minority carriers implanted from a p-type anode region via a p-n junction into an n-type drift layer, and accumulated, when there is forward bias being ejected into the exterior of the semiconductor substrate (an anode electrode and a cathode electrode) when there is reverse bias becomes a reverse current flowing from the cathode side to the anode side. This reverse current is called the diode recovery current (reverse recovery current).
When the diode carries out high speed switching, it is necessary to increase the forward current attenuation rate (di/dt) when switching from a forward bias to a reverse bias. However, when the forward current attenuation rate (di/dt) becomes excessive, it may happen that the diode is destroyed. This is because when there is reverse bias (the anode electrode has negative polarity), carriers (holes) accumulated on the outer peripheral side of the anode region are ejected, because of which a reverse recovery current with high di/dt concentrates in a termination portion, particularly in a corner portion edge, of the anode electrode.
Meanwhile, for a diode used for high speed switching, it is desirable that the value of di/dt resistance, which is the value of the forward current attenuation rate (di/dt) when reaching the destruction, is high. In order to increase the di/dt resistance, it is necessary to suppress as far as possible a concentration of reverse recovery current liable to occur in the vicinity of the outer peripheral end of the anode electrode or anode region when reverse voltage is applied, thereby increasing the reverse recovery current resistance.
As away of doing so, there is publicly known technology whereby recovery resistance (reverse recovery current resistance) is increased by the reverse recovery current being reduced by a low lifetime region being locally formed in an outer peripheral region enclosing a type anode region of the diode, and hastening the elimination of accumulated carriers (JP-A-9-246570). However, there is concern that simply hastening the elimination of the accumulated carriers will lead to a rise in on-state voltage, because of which, in order to control the lifetime of the accumulated carriers without causing the on-state voltage (forward voltage drop) to rise, thereby realizing high speed switching characteristics, there has been a disclosure of technology relating to a method whereby a crystal defect layer region having a carrier recombination center is locally formed by an irradiation with proton, helium, or the like, so as to be a selective region, selective in the depth direction and leaving intervals in a direction parallel to the main surface (JP-A-10-178019). Furthermore, there is known a helium ion irradiation method that enables the fabrication of a lifetime control region restricted to a predetermined range of depths straddling the p-n junction over the whole of a diode chip, or in an edge portion of the anode region. Specifically, there has been disclosed a method whereby the concentration of reverse recovery current is suppressed, and the resistance thereto is increased, by a localized low lifetime layer being formed in a region on both sides straddling the p-n junction plane of a bottom portion of a p+ type diffusion region (anode region), as shown in FIGS. 2 and 3, and shortening the lifetime of electron-hole carriers accumulated in the vicinity of the p-n junction plane of a boundary portion between an active region and a junction termination structure region (JP-A-2001-135831 and JP-A-2005-340528).
Also, a diode having a portion wherein a p-type anode region is extended from an end portion of contact with an anode electrode for a predetermined distance to the chip outer periphery is described in JP-A-2010-50441. There is also a description of a structure wherein another electrode separate from the anode electrode is brought into contact with the anode extension portion, and furthermore, the lifetime of minority carriers in the p-n junction portion of the anode extension portion is shorter than in other portions.